Field of the Invention
The invention relates to a solid electrolyte high temperature fuel cell module and a method for its operation.
It is known that during the electrolysis of water, water molecules are decomposed into hydrogen and oxygen by an electric current. In a fuel cell, that procedure runs in the opposite direction. During the electrochemical joining of hydrogen and oxygen to form water, electric current is produced with a high efficiency, and without the emission of pollutants and carbon monoxide if pure hydrogen is used as a fuel gas. Even when using industrial fuel gases, for example natural gas, and using air instead of pure oxygen, the fuel cell produces distinctly fewer pollutants and less CO.sub.2 than other technologies of fossil energy carriers. The industrial implementation of that principle has led to very different solutions, with various types of electrolytes and operating temperatures between 80.degree. C. and 1000.degree. C.
In the solid electrolyte high temperature fuel cell (Solid Oxide Fuel Cell or SOFC), natural gas is used as the primary energy source. A power density of 1 MW/m.sup.3 enables a very compact construction. Heat which is additionally produced has a temperature of over 900.degree. C.
In the case of a solid electrolyte high temperature fuel cell module, the fuel cell module is also referred to as a "stack" in the technical literature. A window film, a solid electrolyte electrode element, a further window film, a further bipolar plate, and so on, disposed one on another, are provided in that order underneath an upper bipolar covering plate. In that case, a solid electrolyte electrode element lying between two adjacent bipolar plates, including the window films resting directly on both sides of the solid electrolyte electrode element and those sides of each of the two bipolar plates resting on the window films, together form a solid electrolyte high temperature fuel cell.
That type and further types of fuel cell modules are known, for example, from the "Fuel Cell Handbook" by A. J. Appelby and F. R. Foulkes, Van Nostraud Reinhold, pages 442 to 454, or from the article "Brennstoffzellen als Energiewandler" Fuel Cells as Energy Converters!, in Energiewirtschaftliche Tagesfragen, June 1993, issue 6, pages 382 to 390.
German Published, Non-Prosecuted Patent Applications DE 39 35 722 A1 and DE 40 09 138 A1 disclose solid electrolyte high temperature fuel cell modules which include a plurality of solid electrolyte high temperature fuel cells that are connected in series, are planar and rest firmly on one another. A bipolar plate which is installed between directly adjacent cells that are connected in series, electrically conductively connects the cathode of one cell to the anode of the directly adjacent cell and ensures a supply of operating gas through the use of ducts that are let in on both sides. In that case, the ducts conveying the operating gas are disposed parallel to the longitudinal axis of the solid electrolyte high temperature fuel cell module and extend through the entire solid electrolyte high temperature fuel cell module.
A significant problem during the operation of a solid electrolyte high temperature fuel cell module is in dissipating heat produced during the reaction out of the solid electrolyte high temperature fuel cell module. Part of the heat is dissipated through the operating medium. The amount of heat dissipated through the operating medium becomes greater as the temperature difference between the inflowing and outflowing operating media becomes higher. A higher temperature difference can lead to a lower operating temperature of the fuel cells, if the operating media flow into the solid electrolyte high temperature fuel cell module at a lower temperature, which leads to a reduction in the power capacity of the solid electrolyte electrode element.
At the same time, because of the temperature differences in the active cell region, there is the risk that the mechanically sensitive electrolytes will be damaged.
A further problem arises during operation with operating media to be reformed. As a result of the reforming reaction, the region for the feeding in of the operating media is additionally cooled, which likewise leads to mechanical stresses in the electrolytes.
Furthermore, German Published, Non-Prosecuted Patent Application DE 42 17 892 A1 discloses a fuel cell configuration in which a waste gas from a stack chamber, that includes a multiplicity of solid electrolyte fuel cells, is burned outside the stack chamber in a combustion chamber. The heat of the combustion gas is transferred to an oxidation gas and a reaction gas in a heat exchanger connected downstream of the stack chamber, and the gases are subsequently fed to the stack chamber in order to operate the solid electrolyte fuel cells.
A high temperature fuel cell stack with an integrated heat exchanger configuration for the dissipation of the heat released from the same is also disclosed by German Published, Non-Prosecuted Patent Application DE 41 37 968 A1, corresponding to U.S. application Ser. No. 08/245,862, filed May 19, 1994, now abandoned. In that document, a waste gas is heated to the necessary input temperature of the gas turbine within the high temperature fuel cell stack, for further use in a gas turbine connected downstream. In addition, an oxidation gas and a reaction gas for the high temperature fuel cell stack are heated outside the high temperature fuel cell stack, using the heat which is released.